Bitumen extracted from oil sand, such as oil sand mined in the Fort McMurray region of Alberta, is generally made up of water-wet sand grains and viscous bitumen. To eventually produce a commercial petroleum product from oil sand, the bitumen must be removed from the sand. To remove the bitumen from the sand/bitumen mixture, the oil sand is often crushed and then mixed with water to form an oil sand/water slurry. This slurry can then be subjected to what is commonly referred to as “pipeline conditioning” by pumping the slurry some distance through a pipeline, commonly called a hydrotransport pipeline. The conditioned slurry is then typically diluted with a fluid, such as water, to form a diluted slurry. By diluting the slurry, the density of the slurry can be altered to a more desirable density for separation of the bitumen in the slurry. The diluted slurry is then fed to a gravity separation vessel such as a primary separation vessel (PSV) where the relatively quiescent conditions and entrained air in the bitumen allows a significant portion of the bitumen to float towards the top of the gravity separation vessel and collect in a layer of froth, commonly called primary bitumen froth. This primary bitumen froth can be recovered and further treated to eventually be made into a commercial petroleum product.
In addition to the bitumen froth layer, typically a middlings layer and a tailings layer are also formed in the gravity separation vessel. The middlings layer forms below the bitumen froth layer and the tailings layer forms at the bottom of the gravity separation vessel. The middlings and tailings layers are removed and often further treated to extract out additional bitumen that remains in these layers. However, the bitumen in these layers is not as easily recoverable.
To try and increase the quality of the bitumen froth that collects in the bitumen froth layer, an underwash layer is often purposely formed above the middlings layer and below the bitumen froth layer in the PSV. The underwash layer is typically formed by introducing heated liquid, such as water, to the upper portion of the middlings layer and below the bitumen froth layer. The heated liquid in the underwash layer can help to increase the temperature of the bitumen froth produced. The heated underwash water can also replace the middlings in the bitumen froth as it is formed, thereby reducing the amount of solids in the froth.
To enhance gravity separation, quiescent conditions need to be maintained in the PSV. One of the main factors affecting these quiescent conditions is the introduction of the slurry to the gravity separation vessel. Typically, these gravity separation vessels are operated as a continuous process with slurry continuously being introduced into the gravity separation while end products, such as bitumen froth, a tailings stream, etc. are continuously being removed from the gravity separation vessel. The introduction of slurry can have a detrimental effect on these quiescent conditions due to the high velocity of the feed and the recirculation currents formed by the separation of the coarse solids from the slurry. Additionally, the introduction of the slurry can have a detrimental effect on the underwash layer, with swirling and vortices created in the gravity separation vessel by the introduction of the slurry affecting the stability of the underwash layer and causing an erosion of the underwash layer.